Myth vs. Fact Regarding the "Hockey Stick"

Numerous myths regarding the so-called "hockey stick" reconstruction of past temperatures, can be found on various non-peer reviewed websites, internet newsgroups and other non-scientific venues. The most widespread of these myths are debunked below:

MYTH #0:Evidence for modern human influence on climate rests entirely upon the "Hockey Stick" Reconstruction of Northern Hemisphere mean temperatures indicating anomalous late 20th century warmth.

Some proxy-based reconstructions suggest greater variability than others. This greater variability may be attributable to different emphases in seasonal and spatial emphasis (see Jones and Mann, 2004; Rutherford et al, 2004; Cook et al, 2004). However, even for those reconstructions which suggest a colder “Little Ice Age” and greater variability in general in past centuries, such as that of Esper et al (2002), late 20th century hemispheric warmth is still found to be anomalous in the context of the reconstruction (see Cook et al, 2004).

MYTH #2: Regional proxy evidence of warm or anomalous (wet or dry) conditions in past centuries contradicts the conclusion that late 20th century hemispheric mean warmth is anomalous in a long-term (multi-century to millennial) context.

Such claims reflect a lack of awareness of the distinction between regional and large-scale climate change. Similar such claims were recently made in two articles by astronomer Willie Soon and co-authors (Soon and Baliunas, 2003; Soon et al, 2003). These claims were subsequently rebutted by a group of more than a dozen leading climate scientists in an article in the journal “Eos” of the American Geophysical Union (Mann et al, ‘Eos‘, 2003). The rebuttal raised, among other points, the following two key points:

(1) In drawing conclusions regarding past regional temperature changes from proxy records, it is essential to assess proxy data for actual sensitivity to past temperature variability. In some cases (Soon and Baliunas, 2003, Soon et al, 2003) a global ‘warm anomaly’ has been defined for any period during which various regions appear to indicate climate anomalies that can be classified as being either ‘warm’, ‘wet’, or ‘dry’ relative to ’20th century’ conditions. Such a criterion could be used to define any period of climate as ‘warm’ or ‘cold’, and thus cannot meaningfully characterize past large-scale surface temperature changes.

(2) It is essential to distinguish (e.g. by compositing or otherwise assimilating different proxy information in a consistent manner—e.g., Jones et al., 1998; Mann et al., 1998, 1999; Briffa et al., 2001) between regional temperature changes and changes in global or hemispheric mean temperature. Specific periods of cold and warmth differ from region to region over the globe (see Jones and Mann, 2004), as changes in atmospheric circulation over time exhibit a wave-like character, ensuring that certain regions tend to warm (due, for example, to a southerly flow in the Northern Hemisphere winter mid-latitudes) when other regions cool (due to the corresponding northerly flow that must occur elsewhere). Truly representative estimates of global or hemispheric average temperature must therefore average temperature changes over a sufficiently large number of distinct regions to average out such offsetting regional changes. The specification of a warm period, therefore requires that warm anomalies in different regions should be truly synchronous and not merely required to occur within a very broad interval in time, such as AD 800-1300 (as in Soon et al, 2003; Soon and Baliunas, 2003).

MYTH #3: The "Hockey Stick" studies claim that the 20th century on the whole is the warmest period of the past 1000 years.

This is a mis-characterization of the actual scientific conclusions. Numerous studies suggest that hemispheric mean warmth for the late 20th century (that is, the past few decades) appears to exceed the warmth of any comparable length period over the past thousand years or longer, taking into account the uncertainties in the estimates (see Figure 1 in “Temperature Variations in Past Centuries and The So-Called ‘Hockey Stick'”). On the other hand, in the context of the long-term reconstructions, the early 20th century appears to have been a relatively cold period while the mid 20th century was comparable in warmth, by most estimates, to peak Medieval warmth (i.e., the so-called “Medieval Warm Period”). It is not the average 20th century warmth, but the magnitude of warming during the 20th century, and the level of warmth observed during the past few decades, which appear to be anomalous in a long-term context. Studies such as those of Soon and associates (Soon and Baliunas, 2003; Soon et al, 2003) that consider only ‘20th century’ conditions, or interpret past temperature changes using evidence incapable of resolving trends in recent decades , cannot meaningfully address the question of whether late 20th century warmth is anomalous in a long-term and large-scale context.

MYTH #4: Errors in the "Hockey Stick" undermine the conclusion that late 20th century hemispheric warmth is anomalous.

This statement embraces at least two distinct falsehoods. The first falsehood holds that the “Hockey Stick” is the result of one analysis or the analysis of one group of researchers (i.e., that of Mann et al, 1998 and Mann et al, 1999). However, as discussed in the response to Myth #1 above, the basic conclusions of Mann et al (1998,1999) are affirmed in multiple independent studies. Thus, even if there were errors in the Mann et al (1998) reconstruction, numerous other studies independently support the conclusion of anomalous late 20th century hemispheric-scale warmth.

The second falsehood holds that there are errors in the Mann et al (1998, 1999) analyses, and that these putative errors compromise the “hockey stick” shape of hemispheric surface temperature reconstructions. Such claims seem to be based in part on the misunderstanding or misrepresentation by some individuals of a corrigendum that was published by Mann and colleagues in Nature. This corrigendum simply corrected the descriptions of supplementary information that accompanied the Mann et al article detailing precisely what data were used. As clearly stated in the corrigendum, these corrections have no influence at all on the actual analysis or any of the results shown in Mann et al (1998). Claims that the corrigendum reflects any errors at all in the Mann et al (1998) reconstruction are entirely false.

False claims of the existence of errors in the Mann et al (1998) reconstruction can also be traced to spurious allegations made by two individuals, McIntyre and McKitrick (McIntyre works in the mining industry, while McKitrick is an economist). The false claims were first made in an article (McIntyre and McKitrick, 2003) published in a non-scientific (social science) journal “Energy and Environment” and later, in a separate “Communications Arising” comment that was rejected by Nature based on negative appraisals by reviewers and editor [as a side note, we find it peculiar that the authors have argued elsewhere that their submission was rejected due to ‘lack of space’. Nature makes their policy on such submissions quite clear: “The Brief Communications editor will decide how to proceed on the basis of whether the central conclusion of the earlier paper is brought into question; of the length of time since the original publication; and of whether a comment or exchange of views is likely to seem of interest to nonspecialist readers. Because Nature receives so many comments, those that do not meet these criteria are referred to the specialist literature.” Since Nature chose to send the comment out for review in the first place, the “time since the original publication” was clearly not deemed a problematic factor. One is logically left to conclude that the grounds for rejection were the deficiencies in the authors’ arguments explicitly noted by the reviewers]. The rejected criticism has nonetheless been posted on the internet by the authors, and promoted in certain other non-peer-reviewed venues (see this nice discussion by science journalist David Appell of a scurrilous parroting of their claims by Richard Muller in an on-line opinion piece).

The claims of McIntyre and McKitrick, which hold that the “Hockey-Stick” shape of the MBH98 reconstruction is an artifact of the use of series with infilled data and the convention by which certain networks of proxy data were represented in a Principal Components Analysis (“PCA”), are readily seen to be false , as detailed in a response by Mann and colleagues to their rejected Nature criticism demonstrating that (1) the Mann et al (1998) reconstruction is robust with respect to the elimination of any data that were infilled in the original analysis, (2) the main features of the Mann et al (1998) reconstruction are entirely insensitive to whether or not proxy data networks are represented by PCA, (3) the putative ‘correction’ by McIntyre and McKitrick, which argues for anomalous 15th century warmth (in contradiction to all other known reconstructions), is an artifact of the censoring by the authors of key proxy data in the original Mann et al (1998) dataset, and finally, (4) Unlike the original Mann et al (1998) reconstruction, the so-called ‘correction’ by McIntyre and McKitrick fails statistical verification exercises, rendering it statistically meaningless and unworthy of discussion in the legitimate scientific literature.

The claims of McIntyre and McKitrick have now been further discredited in the peer-reviewed scientific literature, in a paper to appear in the American Meteorological Society journal, “Journal of Climate” by Rutherford and colleagues (2004) [and by yet another paper by an independent set of authors that is currently “under review” and thus cannot yet be cited–more on this soon!]. Rutherford et al (2004) demonstrate nearly identical results to those of MBH98, using the same proxy dataset as Mann et al (1998) but addressing the issues of infilled/missing data raised by Mcintyre and McKitrick, and using an alternative climate field reconstruction (CFR) methodology that does not represent any proxy data networks by PCA at all.

When Climatologists Attack!!
The climatologists are angry, and are on the warpath against the industry-funded Tech Central Station: RealClimate » Temperature Variations in Past Centuries and the so-called "Hockey Stick": …coined by the former head of NOAA’s Geo…

In the above article you discuss various myths associated with the “hockey stick” graph. To support your argument you refer to other sources. For example “see Cook et al 2004” at the end of the myth 1 discussion. So I thought – ok, I’ll look up the Cook study and see what it says. Now unfortunately I’m a total non-expert – I only started reading about climate change a few months ago, so it always possible that I’ve misunderstood something. Anyway as I was reading the paper it struck me that neither the Cook study – nor any of the other proxy studies were in total agreement with the hockey stick.

[Response: Your comment is somewhat incorrectly premised. There is no uniquely defined ‘hockey stick’ reconstruction. This term, as defined in our glossary, refers to the general shape, common to a number of independent reconstructions (see this comparison of various proxy- and model-based estimates), characterized by 20th century large-scale warming that exceeds the bounds of the more moderate temperature variations characterizing preceding centuries. The details of any individual reconstruction depend, however, on the regions and seasons represented by that particular reconstruction, as discussed above in the discussion of “myth #1”. In the Cook et al (2004) paper referred to above, the authors are indeed careful to emphasise that their conclusions pertain to the warm-season and extratropical, continental region represented by their particular reconstruction. There are a number of important reasons why reconstructions differ depending on seasonal and spatial sampling considerations (see the response to comment#3 above, and also the discussion in the review paper by Jones and Mann (2004–see section 5.3 therein) [Jones, P.D., Mann, M.E., Climate Over Past Millennia, Reviews of Geophysics, 42, RG2002, doi: 10.1029/2003RG000143, 2004.]. It is therefore not surprising that past summer, extratropical temperature changes, such as are reconstructed by Esper et al (2002) and Cook et al (2004), may indeed have differed significantly from annual temperature changes over the entire (tropical and extratropical) Northern Hemisphere, such as are targeted in other reconstructions (e.g. Jones et al, 1998; Mann et al, 1999; Crowley and Lowery, 2000; Mann and Jones, 2003). Despite these differences in seasonal and spatial emphasis, however, the various reconstructions and model estimates are in fact observed to be consistent within estimated uncertainties. -mike]

You could argue that all the reconstructions show some agreement up to about 1980 – but after that the measured temperature record diverges completely from the proxy records and by the end of the century appears to be at least 0.3 degrees higher than the proxies.

[Response: This is not correct. Most reconstructions only extend through about 1980 because the vast majority of tree-ring, coral, and ice core records currently available in the public domain do not extend into the most recent decades. While paleoclimatologists are attempting to update many important proxy records to the present, this is a costly, and labor-intensive activity, often requiring expensive field campaigns that involve traveling with heavy equipment to difficult-to-reach locations (such as high-elevation or remote polar sites). For historical reasons, many of the important records were obtained in the 1970s and 1980s and have yet to be updated. The fact that a relatively small number of long high-resolution proxy temperature records extend through the most recent decades should not be mis-interpreted as evidence against recent warming (as sometimes done by certain contrarians). The available long records that extend through the present typically do indeed confirm the recent warming. For example, through a composite of the relatively few very long available proxy records that extend all the way through the 1990s, Jones and Mann (2004) show that proxy-reconstructed Northern Hemisphere temperatures [navy blue curve in Figure 2 here, do indeed reproduce the post-1980 warming indicated by the instrumental record (red curve in same figure). -mike]

As I say I thought this was down to me as I hadn’t seen anything – even in sceptic articles which mentioned this. However towards the end of the paper, it refers to the difference between the Cook (or Esper) reconstruction and the observed temperature, saying

“Note that it does very well at tracking the instrumental data on inter-decadal and longer tinescales up to about 1982, after which the tree-ring estimates systematically under-estimate the actual warming. ”

There are probably several possibilities for this, but the two most obvious would be
1. The Surface Temperature is wrong – at least after 1980 or
2. The proxy measurements do not accurately reflect true temperature, i.e. there is a
‘damping’ effect.

[Response: This comment again reflects some degree of misunderstanding regarding what the proxy reconstructions and instrumental data actually show. There is no discrepancy in general between the 20th century trends indicating by the instrumental record and by most proxy reconstructions. In some tree-ring reconstructions (particularly, those based on tree-ring density information, and emphasising higher latitudes), however, anomalous behavior in recent decades apparently related to non-climatic (or, at least, non-temperature related) influences on tree growth does indeed compromise the use of these data in reconstructing temperature changes over the past several decades [see e.g. the discussions by Briffa et al (1998)[Briffa et al, Reduced sensitivity of recent tree-growth to temperature at high northern latitudes, Nature, 391, 678 – 682 (1998)] and also this article in Eos by Mann et al (2003)[Mann, M.E., Ammann, C.M., Bradley, R.S., Briffa, K.R., Crowley, T.J., Hughes, M.K., Jones, P.D., Oppenheimer, M., Osborn, T.J., Overpeck, J. T., Rutherford, S., Trenberth, K.E., Wigley, T.M.L., Response to Comment on ‘On Past Temperatures and Anomalous Late 20th Century Warmth’, Eos, 84, 473, 2003]. The same factors may compromise the behavior of the Esper et al (2002)/Cook et al (2004) reconstructions in recent decades as well. These factors, however, do not appear to be relevant to other, multiple-proxy based reconstructions such as those referred to above. -mike]

Whatever the reason for the divergence, it would seem to suggest that the practice of grafting the thermometer record onto a proxy temperature record – as I believe was done in the case of the ‘hockey stick’ – is dubious to say the least.

“Most proxy reconstructions end somewhere around 1980, for the reasons discussed above….”

Then why does the Cook paper comment on the “under-estimation of actual warming after 1982”. There is a graph (at the top of Page 2) which shows the proxy reconstructions extending well beyond 1980. I have just looked at the Briffa study (also referenced by yourselves) which also shows reconstructions beyond 1980.

[Response: Please carefully read the response to your previous post. Further information is provided in the Jones and Mann (2004) review paper, which is linked to above. Unfortunately, due to other demands on our time, this will have to be the last word on the matter for the present. -mike]

You also say

“While paleoclimatologists are attempting to update many important proxy records to the present, this is a costly, and labor-intensive activity….”

While this may be true I would have thought that this would be seen as a major priority in order to provide confirmation and validation of the surface temperature record. After all, this is the period which is supposed to show ‘unprecedented’ warming surely it’s necessary to obtain as much evidence as possible to confirm this.

There doesn’t seem to be the same problems when it comes to validating (or not) the UAH satellite record.

I accept you know longer wish to discuss this but I would like to comment on part of your previous responses where you say

“…however, anomalous behavior in recent decades apparently related to non-climatic (or, at least, non-temperature related) influences on tree growth does indeed compromise the use of these data in reconstructing temperature changes over the past several decades”

This seems like incredible bad luck. Here we have a perfectly reliable method for reconstructing past temperatures over hundreds of years – then all of a sudden there is “anomalous behavior in recent decades apparently related to non-climatic (or, at least, non-temperature related) influences on tree growth”. And wouldn’t you know it – this anamalous behaviour coincides exactly with a period of ‘unprecedented’ warming.

[Response: These are exactly the types of issues that paleoclimatologists worry about, and spend a great bit of time attempting to deal with. The phenomenon in question gets at the issue of “uniformitarianism” that is that the heart of proxy methods for reconstructing past climates. Proxy-specific biases are, for example, one argument in favor of the use of multiple proxy indicators in reconstructing past climates. A good discussion of all of these issues can be found in the Jones and Mann (2004) review paper referred to in comment #4 above. I hope you find this reference of some help in answering other questions you may have.]

Well, if nothing else, the perturbation of atmosphere and water supplies have certainly been a lot greater in the last 20-50 years than previously. So, as we were saying, we do know that atmospheric composition or the trace gases (everything but O2, N2 and maybe Ar) has changed rapidly in the past half century, maybe a little more, maybe a little less. OTOH, unless you have some information not available to me, these have been relatively stable pre 1900 back at least a few millenia, at least in comparison with recent times.

So why are you surprised that tree growth patterns have altered lately and what would you attribute this to? We do know that northern forests, for example, have been affected by sulphate emissions. I would regard this not as an indicator that the proxy records have been falsified, but that anthropic effects are clearly visible in the recent records.